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<H1><a name="Arguments">13 Argument Handling</a></H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Arguments_nn2">The typemaps.i library</a>
<ul>
<li><a href="#Arguments_nn3">Introduction</a>
<li><a href="#Arguments_nn4">Input parameters</a>
<li><a href="#Arguments_nn5">Output parameters</a>
<li><a href="#Arguments_nn6">Input/Output parameters</a>
<li><a href="#Arguments_nn7">Using different names</a>
</ul>
<li><a href="#Arguments_nn8">Applying constraints to input values</a>
<ul>
<li><a href="#Arguments_nn9">Simple constraint example</a>
<li><a href="#Arguments_nn10">Constraint methods</a>
<li><a href="#Arguments_nn11">Applying constraints to new datatypes</a>
</ul>
</ul>
</div>
<!-- INDEX -->



<p>
In Chapter 5, SWIG's treatment of basic datatypes and pointers was
described.  In particular, primitive types such as <tt>int</tt> and
<tt>double</tt> are mapped to corresponding types in the target
language.  For everything else, pointers are used to refer to
structures, classes, arrays, and other user-defined datatypes.
However, in certain applications it is desirable to change SWIG's
handling of a specific datatype. For example, you might want to
return multiple values through the arguments of a function.  This chapter
describes some of the techniques for doing this.
</p>

<H2><a name="Arguments_nn2">13.1 The typemaps.i library</a></H2>


<p>
This section describes the <tt>typemaps.i</tt> library file--commonly used to
change certain properties of argument conversion.
</p>

<H3><a name="Arguments_nn3">13.1.1 Introduction</a></H3>


<p>
Suppose you had a C function like this:
</p>

<div class="code"><pre>
void add(double a, double b, double *result) {
  *result = a + b;
}
</pre></div>

<p>
From reading the source code, it is clear that the function is storing
a value in the <tt>double *result</tt> parameter.   However, since SWIG
does not examine function bodies, it has no way to know that this is
the underlying behavior.
</p>

<p>
One way to deal with this is to use the 
<tt>typemaps.i</tt> library file and write interface code like this:
</p>

<div class="code"><pre>
// Simple example using typemaps
%module example
%include "typemaps.i"

%apply double *OUTPUT { double *result };
%inline %{
extern void add(double a, double b, double *result);
%}
</pre></div>

<p>
The <tt>%apply</tt> directive tells SWIG that you are going to apply
a special type handling rule to a type. The "<tt>double *OUTPUT</tt>" specification is the
name of a rule that defines how to return an output value from an argument of type
<tt>double *</tt>. This rule gets applied to all of the datatypes
listed in curly braces-- in this case "<tt>double *result</tt>".</p>

<p>
When the resulting module is created, you can now use the function
like this (shown for Python):
</p>

<div class="targetlang"><pre>
&gt;&gt;&gt; a = add(3, 4)
&gt;&gt;&gt; print a
7
&gt;&gt;&gt;
</pre></div>

<p>
In this case, you can see how the output value normally returned in
the third argument has magically been transformed into a function
return value.  Clearly this makes the function much easier to use
since it is no longer necessary to manufacture a special <tt>double
*</tt> object and pass it to the function somehow.
</p>

<p>
Once a typemap has been applied to a type, it stays in effect for all future occurrences
of the type and name.  For example, you could write the following:
</p>

<div class="code"><pre>
%module example
%include "typemaps.i"

%apply double *OUTPUT { double *result };

%inline %{
extern void add(double a, double b, double *result);
extern void sub(double a, double b, double *result);
extern void mul(double a, double b, double *result);
extern void div(double a, double b, double *result);
%}
...
</pre></div>

<p>
In this case, the <tt>double *OUTPUT</tt> rule is applied to all of the functions that follow.
</p>

<p>
Typemap transformations can even be extended to multiple return values.
For example, consider this code:
</p>

<div class="code">
<pre>
%include "typemaps.i"
%apply int *OUTPUT { int *width, int *height };

// Returns a pair (width, height)
void getwinsize(int winid, int *width, int *height);
</pre>
</div>

<p>
In this case, the function returns multiple values, allowing it to be used like this:
</p>

<div class="targetlang"><pre>
&gt;&gt;&gt; w, h = genwinsize(wid)
&gt;&gt;&gt; print w
400
&gt;&gt;&gt; print h
300
&gt;&gt;&gt;
</pre>
</div>

<p>
It should also be noted that although the <tt>%apply</tt> directive is
used to associate typemap rules to datatypes, you can also use the
rule names directly in arguments.  For example, you could write this:
</p>

<div class="code"><pre>
// Simple example using typemaps
%module example
%include "typemaps.i"

%{
extern void add(double a, double b, double *OUTPUT);
%}
extern void add(double a, double b, double *OUTPUT);
</pre></div>

<p>
Typemaps stay in effect until they are explicitly deleted or redefined to something
else.   To clear a typemap, the <tt>%clear</tt> directive should be used.  For example:
</p>

<div class="code">
<pre>
%clear double *result;      // Remove all typemaps for double *result
</pre>
</div>

<H3><a name="Arguments_nn4">13.1.2 Input parameters</a></H3>


<p>
The following typemaps instruct SWIG that a pointer really only holds a single
input value:
</p>

<div class="code"><pre>
int *INPUT
short *INPUT
long *INPUT
unsigned int *INPUT
unsigned short *INPUT
unsigned long *INPUT
double *INPUT
float *INPUT
</pre></div>

<p>
When used, it allows values to be passed instead of pointers.  For example, consider this
function:
</p>

<div class="code"><pre>
double add(double *a, double *b) {
  return *a+*b;
}
</pre></div>

<p>
Now, consider this SWIG interface:
</p>

<div class="code"><pre>
%module example
%include "typemaps.i"
...
%{
extern double add(double *, double *);
%}
extern double add(double *INPUT, double *INPUT);

</pre></div>

<p>
When the function is used in the scripting language interpreter, it will work like this:
</p>

<div class="targetlang"><pre>
result = add(3, 4)
</pre></div>

<H3><a name="Arguments_nn5">13.1.3 Output parameters</a></H3>


<p>
The following typemap rules tell SWIG that pointer is the output value of a
function. When used, you do not need to supply the argument when
calling the function. Instead, one or more output values are returned. 
</p>

<div class="code"><pre>
int *OUTPUT
short *OUTPUT
long *OUTPUT
unsigned int *OUTPUT
unsigned short *OUTPUT
unsigned long *OUTPUT
double *OUTPUT
float *OUTPUT

</pre></div>
<p>
These methods can be used as shown in an earlier example. For example, if you have this C function :</p>

<div class="code"><pre>
void add(double a, double b, double *c) {
  *c = a+b;
}
</pre></div>

<p>
A SWIG interface file might look like this :</p>

<div class="code"><pre>
%module example
%include "typemaps.i"
...
%inline %{
extern void add(double a, double b, double *OUTPUT);
%}

</pre></div>

<p>
In this case, only a single output value is returned, but this is not
a restriction.  An arbitrary number of output values can be returned by applying
the output rules to more than one argument (as shown previously).
</p>

<p>
If the function also returns a value, it is returned along with the argument. For example,
if you had this:
</p>

<div class="code"><pre>
extern int foo(double a, double b, double *OUTPUT);
</pre></div>

<p>
The function will return two values like this:
</p>

<div class="targetlang">
<pre>
iresult, dresult = foo(3.5, 2)
</pre>
</div>

<H3><a name="Arguments_nn6">13.1.4 Input/Output parameters</a></H3>


<p>
When a pointer serves as both an input and output value you can use
the following typemaps :</p>

<div class="code"><pre>
int *INOUT
short *INOUT
long *INOUT
unsigned int *INOUT
unsigned short *INOUT
unsigned long *INOUT
double *INOUT
float *INOUT

</pre></div>

<p>
A C function that uses this might be something like this:</p>

<div class="code"><pre>
void negate(double *x) {
  *x = -(*x);
}

</pre></div>

<p>
To make x function as both and input and output value, declare the
function like this in an interface file :</p>

<div class="code"><pre>
%module example
%include "typemaps.i"
...
%{
extern void negate(double *);
%}
extern void negate(double *INOUT);

</pre></div>

<p>
Now within a script, you can simply call the function normally :</p>

<div class="targetlang"><pre>
a = negate(3);         # a = -3 after calling this
</pre></div>

<p>
One subtle point of the <tt>INOUT</tt> rule is that many scripting languages
enforce mutability constraints on primitive objects (meaning that simple objects
like integers and strings aren't supposed to change).   Because of this, you can't
just modify the object's value in place as the underlying C function does in this example.
Therefore, the <tt>INOUT</tt> rule returns the modified value as a new object
rather than directly overwriting the value of the original input object.
</p>

<H3><a name="Arguments_nn7">13.1.5 Using different names</a></H3>


<p>
As previously shown, the <tt>%apply</tt> directive can be used to apply the <tt>INPUT</tt>, <tt>OUTPUT</tt>, and
<tt>INOUT</tt> typemaps to different argument names.  For example:
</p>

<div class="code"><pre>
// Make double *result an output value
%apply double *OUTPUT { double *result };

// Make Int32 *in an input value
%apply int *INPUT { Int32 *in };

// Make long *x inout
%apply long *INOUT {long *x};

</pre></div>

<p>
To clear a rule, the <tt>%clear</tt> directive is used:
</p>

<div class="code"><pre>
%clear double *result;
%clear Int32 *in, long *x;
</pre></div>

<p>
Typemap declarations are lexically scoped so a typemap takes effect from the point of definition to the end of the
file or a matching <tt>%clear</tt> declaration.
</p>

<H2><a name="Arguments_nn8">13.2 Applying constraints to input values</a></H2>


<p>
In addition to changing the handling of various input values, it is
also possible to use typemaps to apply constraints. For example, maybe you want to
insure that a value is positive, or that a pointer is non-NULL. This
can be accomplished including the <tt>constraints.i</tt> library file.
</p>

<H3><a name="Arguments_nn9">13.2.1 Simple constraint example</a></H3>


<p>
The constraints library is best illustrated by the following interface
file :</p>

<div class="code"><pre>
// Interface file with constraints
%module example
%include "constraints.i"

double exp(double x);
double log(double POSITIVE);         // Allow only positive values
double sqrt(double NONNEGATIVE);     // Non-negative values only
double inv(double NONZERO);          // Non-zero values
int    fclose(FILE *NONNULL);        // Non-NULL pointers only

</pre></div>

<p>
The behavior of this file is exactly as you would expect. If any of
the arguments violate the constraint condition, a scripting language
exception will be raised. As a result, it is possible to catch bad
values, prevent mysterious program crashes and so on.</p>

<H3><a name="Arguments_nn10">13.2.2 Constraint methods</a></H3>


<p>
The following constraints are currently available</p>

<div class="code"><pre>
POSITIVE                     Any number &gt; 0 (not zero)
NEGATIVE                     Any number &lt; 0 (not zero)
NONNEGATIVE                  Any number &gt;= 0
NONPOSITIVE                  Any number &lt;= 0
NONZERO                      Nonzero number
NONNULL                      Non-NULL pointer (pointers only).

</pre></div>

<H3><a name="Arguments_nn11">13.2.3 Applying constraints to new datatypes</a></H3>


<p>
The constraints library only supports the primitive C datatypes, but it
is easy to apply it to new datatypes using <tt>%apply</tt>. For
example :</p>

<div class="code"><pre>
// Apply a constraint to a Real variable
%apply Number POSITIVE { Real in };

// Apply a constraint to a pointer type
%apply Pointer NONNULL { Vector * };

</pre></div>

<p>
The special types of "Number" and "Pointer" can be applied to any
numeric and pointer variable type respectively. To later remove a
constraint, the <tt>%clear</tt> directive can be used :</p>

<div class="code"><pre>
%clear Real in;
%clear Vector *;
</pre></div>

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